1. Field of the Invention
The present invention relates to an optical pickup device comprising an objective lens having a numerical aperture (NA) of 0.75 or more, and an aberration correction element for correcting a spherical aberration generated by a difference of a substrate thickness among first to third optical recording mediums at a time when the objective lens is used, to selectively record on or reproduce from the first to third optical recording mediums different in substrate thickness using first to third laser lights having different wavelengths.
2. Description of the Background Art
In general, optical recording mediums such as a disc-shaped optical disc and card-shaped optical card have been frequently used, because the mediums are capable of recording information signals such as video information, sound information, and computer data in tracks spirally or concentrically formed on a transparent substrate, and capable of accessing a desired track at a high rate during reproduction of recorded tracks.
As this type of optical recording medium, the optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have already been on the market. In recent years, an extra-high density optical disc (Blu-ray Disc) has been actively developed which is capable of recording or reproducing information signals at a density higher than that of CD, DVD in order to increase the density of the optical disc.
First, for the above-described CD, a disc substrate has heretofore been irradiated with a laser beam obtained by focusing a laser beam having a wavelength of around 780 nm with an objective lens having a numerical aperture (NA) of about 0.45 to record or reproduce the information signal on or from a signal surface which is positioned distant from the laser beam incidence surface of the disc substrate by approximately 1.2 mm.
Moreover, for the above-described DVD, the disc substrate has heretofore been irradiated with the laser beam obtained by focusing a laser beam having a wavelength of around 650 nm with an objective lens having a numerical aperture (NA) of about 0.6 to record or reproduce the information signal on or from the signal surface distant from the laser beam incidence surface of the disc substrate by approximately 0.6 mm. In this case, a recording capacity of the DVD is increased six to eight times that of the CD, and the capacity of one surface is about 4.7 gigabyte (GB) with a diameter of the disc substrate of 12 cm.
Furthermore, the above-described extra-high density optical disc has been developed in which the disc substrate is irradiated with a laser beam having a wavelength of 450 nm or less, which has been focused with an objective lens having a numerical aperture (NA) of 0.75 or more, so as to be capable of recording or reproducing the information signal on or from a signal surface distant from the laser beam incidence surface by approximately 0.05 mm to 0.15 mm. In this case, the recording capacity of the surface of the extra-high density optical disc is around 25 gigabytes (GB) with the diameter of the disc substrate of 12 cm.
Additionally, there has been an optical head device which records or reproduces three types of optical discs having different disc substrate thicknesses (see Japanese Patent Application Laid-Open No. 2003-67972 (page 19, FIG. 8), for example).
FIG. 1 is a diagram showing a configuration of a conventional optical head device. FIGS. 2A to 2C are diagrams schematically showing that three types of optical discs are irradiated with an emission light from three types of optical systems in the conventional optical head device.
A conventional optical head device 110 shown in FIG. 1 is described in the Japanese Patent Application Laid-Open No. 2003-67972 (page 19, FIG. 8) and document “Blue/DVD/CD compatible head., MICROOPTICS NEWS Vol. 20, No. 3, Micro-optics Research Group Journal, 2002.9.6.” (pages 20 and 21, FIGS. 2, 7), and here the device will be briefly described with reference to these documents.
As shown in FIG. 1, the conventional optical head device 110 comprises first to third optical systems 111 to 113 for three types of optical discs 101 to 103, first and second interference filters 114, 115, a wavelength selection filter 116, and an objective lens 117.
Each of the three types of first to third optical systems 111 to 113 comprises a semiconductor laser, and a photodetector for receiving a reflected light from the optical disc. In this case, the wavelength of the semiconductor laser in the first optical system 111 is 405 nm, the wavelength of the semiconductor laser in the second optical system 112 is about 650 to 660 nm, and further the wavelength of the semiconductor laser in the third optical system 113 is about 780 to 785 nm.
The first interference filter 114 has a function of transmitting a light having a wavelength of 405 nm, and reflecting a light having a wavelength of 650 to 660 nm. The second interference filter 115 has a function of transmitting a light having a wavelength of 405 nm and a light having a wavelength of 650 to 660 nm, and reflecting a light having a wavelength of 780 to 785 nm.
Moreover, the emitted light from the semiconductor laser in the first optical system 111 is transmitted through the first and second interference filters 114, 115 in order. As shown in FIG. 2A, the light in a state of parallel light is incident upon and subsequently transmitted through the wavelength selection filter 116 as such, incident upon the objective lens 117, and converted onto the optical disc 101 having a disc substrate thickness of 0.1 mm and having next-generation standards. Thereafter, conversely, the reflected light from the optical disc 101 returns and is received by a photodetector in the first optical system 111.
Moreover, the emitted light from the semiconductor laser in the second optical system 112 is reflected by the first interference filter 114 and transmitted through the second interference filter 115. As shown in FIG. 2B, the light in a state of divergent light is incident upon and subsequently diffracted by the wavelength selection filter 116, incident upon the objective lens 117, and converted onto the optical disc 102 having a disc substrate thickness of 0.6 mm and having DVD standards. Thereafter, conversely, the reflected light from the optical disc 102 returns and is received by the photodetector in the second optical system 112.
Furthermore, the emitted light from the semiconductor laser in the third optical system 113 is reflected by the second interference filter 115. As shown in FIG. 2C, the light in a state of divergent light is incident upon and subsequently diffracted by the wavelength selection filter 116, incident upon the objective lens 117, and converted onto the optical disc 103 having a disc substrate thickness of 1.2 mm and having CD standards. Thereafter, conversely, the reflected light from the optical disc 103 returns and is received by the photodetector in the third optical system 113.
According to the conventional optical head device 110 constituted as described above, a spherical aberration generated by the difference of the disc substrate thickness is corrected by the wavelength selection filter 116, so that three types of optical discs 101 to 103 can be recorded or reproduced.
Additionally, in the conventional optical head device 110 described in the Japanese Patent Application Laid-Open No. 2003-67972 and the document “Blue/DVD/CD compatible head, MICROOPTICS NEWS Vol. 20, No. 3, Micro-optics Research Group Journal, 2002.9.6.”, the emitted lights from the respective semiconductor lasers in the first to third optical systems 111 to 113 are incident upon the wavelength selection filter 116 and objective lens 117 in order. In this case, as shown in FIGS. 2A to 2C, the emitted light from the semiconductor laser in the first optical system 111 is incident upon the wavelength selection filter 116 in the state of the parallel light, but the respective emitted lights from the semiconductor lasers in the second and third optical systems 112, 113 are incident upon the wavelength selection filter 116 in a state of divergent light. Here, when the divergent light is incident upon the wavelength selection filter 116, and when an optical axis of the divergent light deviates from that of the objective lens 117, the spherical aberration is remarkably deteriorated as compared with the parallel light, and it is difficult to adjust the optical axis at an assembly time as compared with the parallel light.
Moreover, in the document “Blue/DVD/CD compatible head, MICROOPTICS NEWS Vol. 20, No. 3, Micro-optics Research Group Journal, 2002.9.6.”, when the first to third optical systems 111 to 113 are assembled, an objective lens shift by the shifts of the optical axes of the respective optical systems 111 to 113 and the objective lens 117 is considered, or a comatic aberration generated by the objective lens shift is corrected. In this case, an RMS wave front aberration (γ rms.) with respect to the objective lens shift (μm) is described as shown in FIG. 3.
FIG. 3 is a diagram showing the wave front aberration at the time of the objective lens shift with respect to the DVD and CD in the conventional optical head device.
In FIG. 3, in general, since an allowable error of ±300 μm from a central axis of the objective lens 117 with respect to the objective lens shift is desired, data is described in this range. However, in either DVD or CD, the wave front aberration is small in a case where there is not any objective lens shift. However, when the objective lens shift is large because of a large absolute value of a magnification (incident conjugated length is short), the wave front aberration rapidly increases. It is to be noted that the incident conjugated length is an interval between the objective lens and a laser light source (additionally, in a case where there is not any optical element between the objective lens and laser light source).
In this case, judging from the figure, when the objective lens shift exceeds approximately ±150 μm, a known Marechal criterion of 0.07λ rms. is exceeded, and it can be confirmed that the device is not practical.